var pwdlevel_val =

 "{"minLength":1,"maxLength":20,"lowercase":false,"uppercase":false,"number":false,"special":false,"limitday":"","maximum":""}";

var jsondata = $.parseJSON(pwdlevel_val);

var minLength = jsondata.minLength;

var maxLength = jsondata.maxLength;

...

var pwdlevel_val =

 "{'minLength':1,'maxLength':20}";

pwdlevel_val = pwdlevel_val.replace(/\'/g,"\"");//g,表示全部替换。